Consider converting codebase to async

[briantist]

Probably a big project but not sure.

Might be a good time to look at converting from Flask to FastAPI.
We could also look more seriously at using antsibull-core as an upstream galaxy client.

Since we rely so much on dohq_artifactory though, it may end up not being that useful. because that library is not async, and our slowest calls are probably all in that library.

We could implement our own artifactory client instead. I don't really want to do that, but the APIs we need are quite limited. Not needing to follow pathlib semantics would also be nice, much easier to do testing, gains in efficiency...

Not really planning to do this any time soon but recording some thoughts.